Project Summary/Abstract The long-term goal of the proposed research is to elucidate the molecular mechanisms by which mutations in genes that regulate rhodopsin trafficking and photoreceptor membrane renewal lead to retinal diseases. Mutations in rhodopsin that affect the C- terminal VxPx motif cause severe forms of autosomal dominant retinitis pigmentosa (ADRP). We identified and defined rhodopsin C-terminal VxPx motif as the conserved ciliary targeting signal. Furthermore, we identified a targeting complex that recognizes this signal and regulates rhodopsin sorting into post-Golgi transport carriers (RTCs). The targeting complex is assembled upon direct binding of the small GTPase Arf4 to the VxPx motif. As the VxPx motif is present in other ciliary membrane proteins, some of which bind Arf4, this leads us to propose that membrane targeting to the rod outer segments (ROS) is equivalent to ciliary targeting, and that the complex we identified is a ciliary targeting complex. We plan to study the role of this complex in rhodopsin trafficking and photoreceptor polarity. We propose: 1. To elucidate the architecture of the targeting complex, which is comprised of the small GTPases Arf4 and Rab11, the Arf GAP ASAP1, and the Arf/Rab11 effector FIP3, and determine its mode of interaction with rhodopsin at the trans-Golgi network (TGN), which is essential for ciliary targeting; 2. To determine whether the ASAP1/Rab11/FIP3 complex that remains associated with RTCs serves to recruit regulators of RTC fusion; and 3. To delineate the individual roles of the components of the complex in ciliary targeting of rhodopsin. To accomplish this, we will use our established cell-free system that reconstitutes rhodopsin trafficking in vitro and probe molecular interactions by biochemical assays, co-immunoprecipitation and pull-down experiments with recombinant and purified components of the complex, as well as by transgenic expression of inhibitory and dominant-negative mutant proteins, and by specific protein knockdowns and expression of mutant proteins in ciliated epithelial cells transfected with rhodopsin-eGFP-VxPx fusion protein. The product of our program will be the basic understanding of the molecular linkage between the regulatory machinery involved in the renewal of light-sensitive membranes through ciliary targeting and a wide range of systemic cilia disorders. This is expected to provide the foundation for improved therapeutic strategies for treatment of retina-specific and syndromic forms of photoreceptor loss.